WHITE PAPER: Periodically poled lithium niobate used in spectroscopy

Introduction

Spectroscopy is the study of the absorption and emission of light and other radiation by matter, as related to the dependence of these processes on the wavelengths of radiation. It plays a vital role in various scientific and industrial fields by providing valuable insights into the composition, structure, and dynamics of materials. Spectroscopy techniques are employed in a wide range of applications, including chemistry, physics, biology, environmental science, materials science, pharmaceuticals, and many others. The global spectroscopy market size reached $16 billion (USD) in the year 2022 and is expected to hit around $31 billion (USD) by 2032 with increased utilization of spectroscopy methods for testing purposes.  There are rising demands from the laboratories for newly developed technology and the market is growing at a CAGR of 7.3% ^[1].

Efficiency and versatility are critical factors in spectroscopy, as researchers strive to obtain accurate and detailed information in a timely manner. To meet these demands, there is a constant need for innovative and advanced spectroscopy techniques that offer enhanced sensitivity, broad tunability, and improved signal generation. Periodically poled lithium niobate (PPLN) has emerged as a valuable platform for various spectroscopy techniques, enabling efficient and versatile manipulation of light-matter interactions. By exploiting its unique properties and periodic poling structure, PPLN offers enhanced wavelength conversion capabilities, broad tunability, and increased detection sensitivity. This white paper explores the potential of PPLN in spectroscopy applications, including nonlinear optical spectroscopy, sum frequency generation spectroscopy, and raman spectroscopy.

Properties of PPLN for Spectroscopy

PPLN offers a large nonlinear coefficient, enabling efficient wavelength conversion and generation of nonlinear optical signals. Its wide transparency range covers a broad spectral range, allowing researchers to access different regions of the electromagnetic spectrum. Moreover, PPLN exhibits a high damage threshold, making it capable of handling intense laser beams. These properties, combined with the flexibility of PPLN in achieving quasi-phase matching conditions through periodic poling, make it an attractive material for spectroscopy applications.

Large Nonlinear Coefficient: PPLN possesses a large nonlinear coefficient. The highest nonlinear coefficient is d33=25pm/V, which corresponds to interactions that are parallel to the z-axis, i.e. type 0 phase matching. For periodically poled MgO:LN, the effective nonlinear coefficient d_eff is typically 14pm/V, which is much higher than traditional nonlinear crystals such as lithium triborate (LBO, 0.85pm/V), beta barium borate (BBO, 2.5pm/V) or potassium titanyl phosphate (KTP, 3.4pm/V). This property allows efficient frequency conversion processes, such as second-harmonic generation (SHG), sum frequency generation (SFG), difference frequency generation (DFG), and optical parametric oscillation (OPO), optical parametric amplification (OPA), which are essential for various spectroscopic applications.

Wide Transparency Range: PPLN exhibits a broad transparency range that extends from the ultraviolet (UV) to the mid-infrared (mid-IR) spectrum. The transparency window covers wavelengths from approximately 380nm to 5μm. This wide transparency range enables PPLN to be utilized for spectroscopic studies across a broad range of wavelengths.

High Damage Threshold: PPLN has a high damage threshold, allowing it to withstand high-intensity laser radiation. This property is crucial for spectroscopy applications that involve intense laser beams, as it ensures the crystal’s stability and longevity under demanding operating conditions. For femtosecond laser source, PPLN could handle up to 8GW/cm² power intensity. 

Flexibility in Wavelength Conversion: PPLN can be engineered to exhibit a quasi-phase-matching (QPM) structure by periodically poling the crystal. This process creates a series of alternating regions with opposite polarization orientations. By selecting the appropriate poling period, the QPM wavelength can be adjusted to match specific application requirements. This flexibility in wavelength conversion enables efficient and precise tuning of generated or converted frequencies for spectroscopic investigations.

Broad Tunability: The QPM structure of PPLN allows for broad tunability of the generated or converted wavelengths. By adjusting the temperature of the crystal, the phase-matching condition can be tailored, leading to tunable output across a wide range of wavelengths. This tunability is advantageous for spectroscopy techniques that require the ability to scan or access different wavelengths.

PPLN Used in Spectroscopy

Two-Photon Absorption Spectroscopy is a nonlinear optical technique that involves the simultaneous absorption of two photons by a molecule or material. The process occurs when the combined energy of two lower-energy photons matches the energy required for an electronic transition that would typically require a single higher-energy photon. In this technique, a pulsed laser source with a relatively long pulse duration and a near-infrared (NIR) wavelength is used to excite the sample. The longer pulse duration helps to ensure that two photons are absorbed simultaneously, leading to fluorescence emission or other measurable signals. PPLN is employed in two-photon absorption spectroscopy as a frequency doubler to convert the NIR laser wavelength to a shorter wavelength, typically in the visible range^[2]. PPLN’s large nonlinear coefficient and flexibility in wavelength conversion enable efficient SHG of the NIR laser light. By exploiting the QPM property of PPLN, the conversion efficiency can be significantly enhanced, resulting in a stronger and more detectable signal for two-photon absorption spectroscopy^[3].

Sum Frequency Generation (SFG) spectroscopy is a powerful and non-linear optical technique used to study the surface and interface properties of materials^[4]. It provides valuable information about molecular vibrations and interactions at interfaces, which are essential for understanding the behavior of surfaces, thin films, and interfaces in various applications, such as catalysis, bio interfaces, and materials science. SFG spectroscopy involves two incident photons interacting to generate a new frequency (sum frequency) equal to the sum of the frequencies of the two incident photons. SFG is highly surface-specific and can selectively probe the molecular vibrations at the interface or surface without interference from the bulk material. This makes it particularly suitable for studying molecular structures and dynamics at buried interfaces. SFG spectroscopy provides vibrational information about the molecules present at the interface, enabling researchers to study molecular orientations, hydrogen bonding, and other interactions. SFG Spectroscopy is a non-destructive technique that can probe surfaces and interfaces without altering the sample. And it’s highly sensitive to molecular structures and orientations, allowing researchers to study monolayers or very thin films. The surface-selective nature of SFG makes it an ideal tool to study buried interfaces, such as those found in bio-membranes or electrode-electrolyte interfaces. SFG can be combined with time-resolved measurements, allowing researchers to study dynamic processes at interfaces on a femtosecond timescale^[5].

PPLN is good material to generate coherent SFG signals with high sensitivity and tunability. PPLN crystals can be engineered with different periodicities, enabling tunability of the generated SFG signal over a wide range of frequencies. The quasi-phase-matching condition in PPLN greatly enhances the efficiency of the SFG process, resulting in stronger and more easily detectable signals. PPLN crystals can be designed to cover a broad range of infrared and visible wavelengths, making them compatible with a variety of laser sources. And PPLN provides coherent SFG signals, allowing for phase-sensitive measurements and various coherence-based spectroscopic techniques. Due to the high conversion efficiency, PPLN-based SFG setups can achieve excellent sensitivity, enabling the study of monolayers or weakly interacting interfaces.

PPLN also has shown great potential in enhancing Raman spectroscopy, a widely used technique for molecular identification and analysis. Raman spectroscopy provides valuable information about molecular vibrations and chemical composition, but it often suffers from weak signals, limiting its sensitivity and applicability in certain scenarios. PPLN can overcome these limitations and enhance Raman signals through processes such as Stimulated Raman Scattering (SRS) and Coherent anti-Stokes Raman Scattering^[6]. SRS is a non-linear optical process that can significantly amplify weak Raman signals by using a powerful pump laser to stimulate Raman transitions. The process involves the interaction of the pump laser with the sample, leading to an amplification of the Raman signal at a different frequency. PPLN can be employed as a nonlinear medium for SRS due to its unique property of quasi-phase matching, which allows efficient energy conversion from the pump to the Raman-shifted signal. Researchers used PPLN crystals to build all-solid-state laser system for SRS microscopy. They demonstrated SRS microscopy at a 30-µs pixel dwell time with high chemical contrast, signal-to-noise ratio in excess of 45 and no need for balanced detection^[7].

Coherent Anti-Stokes Raman Scattering (CARS) Spectroscopy is a powerful nonlinear optical technique used for label-free chemical imaging and vibrational spectroscopy. It allows the detection and characterization of molecular vibrations by exploiting the Raman scattering phenomenon. CARS spectroscopy involves the interaction of three laser beams: a pump beam, a Stokes beam, and a probe beam. The pump and Stokes beams are combined to generate a coherent anti-Stokes signal at a lower frequency, which is then detected using the probe beam. The frequency difference between the pump and Stokes beams corresponds to a vibrational mode of interest. A PPLN-based OPA system could amplify the emitted imaging signal from SHG and CARS microscopy imaging, and the amplified optical signal is strong enough to be detected by a biased photodiode under ordinary room light conditions^[8].

PPLN for Wavelength Conversion

Second harmonic generation (SHG), or frequency doubling, is the most commonly used second order non-linear process. In SHG, two input pump photons with the same wavelength λ_P are combined through a nonlinear process to generate a third photon at λ_SHG, where λ_SHG = λ_P/2 (or in terms of frequency f_SHG = 2f_P).

MgO:PPLN SHG crystals can be fabricated with QPM grating periods suitable for a wide range of commercially available pump laser wavelengths from 976 nm to 2100 nm, allowing generation of frequency doubled light between 488nm and 1050nm.

Sum frequency generation (SFG) combines two input photons at λ_P and λ_S to generate an output photon at λ_SFG , where λ_SFG = (1/ λ_P + 1/ λ_S)^-1 (or in terms of frequency f_SHG = f_P + f_S).

By combining readily available fixed (e.g. 1550nm) and tunable (e.g. 780/810nm) pump laser sources MgO:PPLN SFG crystals can provide tunable output light between 500-700nm.

Difference frequency generation (DFG) occurs when two input photons at λ_P and λ_S are incident on the crystal, the presence of the lower frequency signal photon λ_S, stimulates the pump photon λ_P, to emit a signal photon λ_S and idler photon at λ_i , where λ_i = (1/ λ_P – 1/ λ_S)^-1 (or in terms of frequency f_i = f_P – f_S). In this process, two signal photons and one idler photon exit the crystal resulting in an amplified signal field. This is known as optical parametric amplification(OPA). Furthermore, by placing the nonlinear crystal within an optical resonator, also known as an optical parametric oscillator(OPO), the efficiency can be significantly enhanced.

References

1. https://www.precedenceresearch.com/spectroscopy-market.
2. D. Xu, et. al , “Widely-tunable synchronisation-free picosecond laser source for multimodal CARS, SHG and two-photon microscopy,” Biomedical Optics Express , vol. 12, no. 2 , p. 1010, 2021.
3. H. He, et, al. , “Deep-tissue two-photon micrescopy with a frequency-doubled all-fiber mode-locked laser at 937nm,” Advanced Photonics Nexus, vol. 1(2), p. 026001, 2022.
4. A. Morita, Theory of Sum Frequency Generation Spectroscopy, Springer.
5. A. Ghosh, et. al, , “Femtosecond time-resolved and two-dimensional vibrational sum frequency spectroscopic instrumentation to study structural dynamics at interfaces,” Review of Scientific Instruments , vol. 79, p. 093907, 2008.
6. D. Polli, et. ac, “Broadband Coherent Raman Scattering Microscopy,” Laser & Photonics reviews , vol. 12, no. 9, 2018.
7. T. Steinle, et. al. , “Synchronization-free all-solid-state laser system for stimulated Raman scattering microscopy,” Light: Science & Applications, vol. 5, 2016.
8. Y. Sun, et al. , “Nonlinear optical imaging by detection with optical parametric amplification (invited paper),” Journal of Innovative Optical Health Sciences, vol. 16, no. 1, p. 2245001, 2023.